It has long been known that the effective resistivity of certain metals was sometimes substantially eliminated when the metal was exposed to low temperature conditions. Of particular interest were the metals and metal oxides which can conduct electricity under certain low temperature conditions with virtually no resistance. These have become known as superconductors. Certain metals, for example, are known to be superconductive when cooled to about 4.degree. on the Kelvin scale (.degree.K.), and certain niobium alloys are known to be superconductive at about 15.degree. K., some as high as about 23.degree. K.
Discovery of superconductivity in the system La-Ba-Cu-O (J. G. Bednorz and K. A. Muller, Zeit. Phys. B 64, 189-193 [1986]) and in the system Y-Ba-Cu-O (Wu et al, Phys. Rev. Lett. 58, 908-910 [1987]) has stimulated the search for other systems, particularly with a view to substituting other elements for the rare earths (RE) used in the earlier materials. For example, replacement of RE by Bi and Tl has been reported (papers in press). In preparing the system Tl-Ba-Cu-O, Z. Z. Sheng and A. M. Hermann (Superconductivity in the Rare Earth-Free Tl-Ba-Cu-O System above Liquid Nitrogen Temperature) (communication from the authors), first mixed and ground BaCO.sub.3 and CuO to obtain a product which they heated, then intermittently reground to obtain a uniform black Ba-Cu-Oxide powder, which was then mixed with Tl.sub.2 O.sub.3, ground, and heated, with formation of a superconducting material. It was noted that the Tl oxide partially melted and partially vaporized.
The superconductor system Tl-Ca-Ba-Cu-O was also reported in a paper by Sheng and Hermann, "Bulk Superconductivity at 120K in the Tl-Ca-Ba-Cu-O System" (communication from the authors). The authors reported "stable and reproducible bulk superconductivity above 120K with zero resistance above 100K". According to the paper the composition was prepared by mixing and grinding together Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4. The ground mixture was pressed into a pellet and heated in flowing oxygen. The result was cooled and found to be superconducting.
See also the paper by Hazen et al, "100K Superconducting Phases in the Tl-Ca-Ba-Cu-O System" (communication from the authors) which refers to two superconducting phases, Tl.sub.2 Ca.sub.2 Ba.sub.2 Cu.sub.3 O.sub.10+.delta. and Tl.sub.2 Ca.sub.1 Ba.sub.2 Cu.sub.2 O.sub.8+.delta., both with onset T.sub.c near 120K and zero resistivity at 100K. Preparation included grinding together Tl.sub.2 O.sub.3, CaO, and BaCu.sub.3 O.sub.4 (or Ba.sub.2 Cu.sub.3 O.sub.5), followed by heating.
And see "Nota Bene" in High T.sub.c Update, vols. 2, No. 6, p.1, Mar. 15, 1988, further re properties of the Tl-Ca-Ba-Cu-O system.
Wang et al, Comparison of Carbonate, Citrate, and Oxalate Chemical Routes to the High-T.sub.c Metal Oxide Superconductors La.sub.2-x Sr.sub.x CuO.sub.4, Inorg. Chem. 26, 1474-1476 (1987) discloses a carbonate precipitation technique. The precipitant was K.sub.2 CO.sub.3. According to the paper, it was necessary to wash the precipitate repeatedly, an obvious disadvantage in production work. Washing was necessary because potassium adversely affects superconductivity properties of the finished material. If I wash repeatedly in the co-precipitated carbonate process (below described) I remove barium, a highly detrimental loss in my process.
From the technical viewpoint it may seem obvious that co-precipitated carbonates would provide enhanced homogeneity. However, the technical solution to the problem encounters serious difficulties. Thus, the Wang et al process, using potassium carbonate (or sodium carbonate) necessitated numerous washings and apparently left detectable amounts of alkali in the ceramic base even so. As noted, serial washings remove Ba, and would be unworkable in my process. Nor is it merely sufficient that the carbonate be derived from a cation that would burn off completely. For example, ammonium carbonate does not work, because a pH below 7 is required to prevent formation of copper tetrammine, but under these conditions bicarbonate ion is formed, with consequent formation of barium bicarbonate, which, being slightly soluble disrupts the desired stoichiometry. Quaternary ammonium carbonates, on the other hand, form the desired metal carbonates simply and cleanly without troublesome side-formation of complexes or coordination compounds, with firm and precise retention of the intended stoichiometry. The coated particles are readily recovered for further processing.
So far as I have been able to determine, two known superconductors have never before been reacted together.
The inventive concept is not limited to the examples presented here. It is expected to extend to duos, triads, etc., of other superconductors with formation of new compounds. Materials considered within the inventive concept include the oxide system L-M-A, where L is at least one trivalent metal, including Bi, Sc, V, and other rare earths; M is at least one bivalent metal, including Ba, Be, Mg, Ca, and Sr; and A is at least one metal of multiple valency including Nb, Cu, Ag, and Au.
These may be reacted in the "melt", or the total mix may be formed by carbonate coprecipitation, as may be feasible and appropriate. It is not required that all the initial materials be superconductors.